A liquid crystal display device acting as a video display device illuminates a liquid crystal panel with an LED backlight including light emitting diodes (LEDs).
Particularly, for such a video display device, a technique called “local contrast control” is known (e.g., Japanese Patent Laid-Open No. 2001-142409). In this technique, LEDs are two-dimensionally arranged directly under a liquid crystal panel and the brightness of the LED is controlled according to the feature quantity of a video signal, mainly an intensity value. The local contrast control can improve the contrast of a displayed image.
In recent years, video display devices (hereinafter, will be called 3D-TVs) with a 3D (Three Dimension) display function have been announced one after another. A 3D-TV displays a left-eye image and a right-eye image at the same time or in a time sharing manner, allowing a viewer to recognize a three-dimensional image. The former is called an image segmentation system while the latter is called a time-sharing system.
Local contrast control is expected to be introduced to 3D-TVs in the future. In this case, a feature quantity needs to be detected for a left-eye image and a right-eye image. Particularly, in the time-sharing system, the intensity of a backlight needs to be alternately switched between the left-eye image and the right-eye image based on the set intensity value of an LED backlight, the set value being determined by the feature quantity.
FIG. 14 illustrates a typical configuration of a 3D-TV of the time-sharing system with a local contract control function. FIG. 14 illustrates a liquid crystal display device that displays full high definition (FHD) images for left and right eyes in a time sharing manner with a frequency of 120 Hz. Shutter glasses synchronized with the switching of left and right displayed images are worn to enable stereoscopic vision.
As shown in FIG. 14, a video display device 1000 receives FHD video signals of two systems: a right-eye video signal 1001a and a left-eye video signal 1001b. A video signal transmitted in a 3D transmission format of High-Definition Multimedia Interface (HDMI) is outputted after undergoing expansion, I/P (Interlace/Progressive) conversion, and so on in a circuit (not shown) preceding the configuration of FIG. 14. FIG. 15A shows the state of the output. As shown in FIG. 15A, the right-eye video signal 1001a and the left-eye video signal 1001b are in-phase signals (simultaneously inputted) with a frame period of 60 Hz. A liquid crystal drive unit 1002 displays an image on a liquid crystal panel 1003 based on the video signals 1001 (1001a, 1001b). The liquid crystal panel 1003 is illuminated from the back by an LED backlight 1005 driven by an LED driver 1004. In this configuration, the video display device 1000 includes a signal processing device 1010 that outputs a brightness control signal to the LED backlight 1005 that illuminates the liquid crystal panel 1003 with light from the back. The signal processing device 1010 includes two local control units 1006 (right-eye local control unit 1006a and left-eye local control unit 1006b) that determine the light quantity of an LED for illuminating an image according to a feature quantity for each of right and left eyes, and a selector (selecting unit) 1007 that transmits results determined by the local control units 1006, to the LED driver 1004 while switching the results every 120 Hz. As shown in FIG. 15B, a displayed image and an inputted image are provided at different times, and thus the signal processing device 1010 of the video display device 1000 further includes a delay adjusting memory 1008 that absorbs the difference. Hence, when the liquid crystal panel 1003 displays a right-eye image, the LED backlight 1005 is illuminated according to the feature quantity of the right-eye video signal 1001a in the previous frame. When a left-eye image is displayed, the LED backlight 1005 is illuminated according to the feature quantity of the left-eye video signal 1001b in the previous frame.